1. Field of Invention
The invention relates to a picture image input method which optically inputs a picture image of an original medium.
2. Description of Related Art
A picture image input apparatus (scanner) optically inputs and photoelectrically converts a picture image of an original medium, and outputs this picture image as electronic data.
When the picture image of an original medium is input into this type of scanner, a look-up table is necessary in order to reproduce the object image. The look-up table, for example, merges a table used to revise variances characteristic of the apparatus and a table used for picture image adjustment according to commands from the user.
In the picture image input of a negative film (negative original medium) which is a transmissive original medium, a negative gradation conversion table, which converts the negative to a positive, is also merged into the look-up table.
Furthermore, in the picture image input of a negative film, a prescan must be performed in order to create the negative gradation conversion table. With the exception of creating the negative gradation conversion table, the prescan performs the same operations as the normal scan during picture image input.
The scanner can be connected to a host computer. Thus, the negative gradation conversion table can be created initially by commands from the host computer.
The operation of the scanner is discussed below. First, the scanner receives from the host computer the resolution, the picture image input range and the data in the look-up table. The linear negative gradation conversion table, which is the basis, is merged into the look-up table. The scanner, upon receiving from the host computer a command to start scanning, starts the prescan using the look-up table.
Next, the scanner creates a histogram of the brightness of the input picture image and the frequency thereof, as shown in FIG. 5a, from the picture image data input through the prescan.
The shadow point used to match the white point is found from the histogram. The white point is the location of the maximum value of the output of the negative gradation conversion table. In addition, the shadow point is the darkest location on the negative film, and is the brightest location of the original object. That is to say, the white point is the brightest location of the original object, and is the point which should be the brightest location in the data from the negative-to-positive conversion of the picture image which was input.
For example, from the histogram shown in FIG. 5a, the brightness value is added in the high direction from level 0. The value immediately prior to the cumulative frequency value which crosses 0.3% (cumulative frequency coefficient 1) of the total is made the shadow point S.
Conversely, the frequency value is added from the maximum level (4095 in the case of a 12-bit table) in the low direction. The value immediately prior to where the cumulative frequency value crosses 0.03% (cumulative frequency coefficient 2) of the total is made the highlight point H.
A negative gradation conversion table, in which the highlight point and the shadow point have been determined as described above, is shown in FIG. 5b. 
In the scanner, the negative gradation conversion table obtained as described above is merged into the look-up table. The picture image data input by the actual picture image input is converted using this table and is sent to the host computer.
However, when the highlight point of the negative gradation conversion table is found as described above, the black level in the original object photographed on the negative film may not be correctly set.
For example, transparent portions, where the original medium picture image does not exist, may be included in the orange on which prescanning is performed. Thus, as shown in FIG. 5a, in the histogram which is obtained, a region 52 indicating a brightness distribution is created outside region 51 which indicates the correct brightness distribution.
In this state, when the highlight point is set as described above, the frequency is added from the maximum level of the region 52 in the low direction. The value immediately prior to the cumulative frequency value which crosses 0.03 of the total is utilized. Here, there is a large difference from the highlight point obtained from the region 51 which indicates the brightness distribution in the actual picture image.
Consequently, for example in the picture image input using the look-up table wherein the level of black of the object on the negative film is not correctly set, the picture image data which is obtained is too bright overall. This results in a picture image in which black is not well defined.
In addition, because of the influence of the colors of the film base of the negative film, the image is reproduced with a bluish coloration which is different from that of the actual original picture image.